Elevated structures on the seabed built by calcareous organisms. The most important reef-building organisms are stony corals. Most reefs occur in shallow tropical marine environments and these are characterized by high rates of carbonate accumulation. Coral reefs grow in warm, clean, shallow waters along mainly tropical coastlines. They don't occur or disappear where water temperatures are below 14 ° C; at depth where the sunlight does not penetrate; where there is freshwater run-off, and where the water is muddy. There are fringing reefs, barrier reefs and atolls. Non coral reefs could be built by gastropods in combination with coralline algae, oysters, polychaete worms and Sabellaria.